Turbomachines use numerous sealing techniques to establish seals between stationary and rotating parts of the turbine. In some instances, the seals are designed to provide radial movement away from the rotating components to prevent rubs during operation. In other instances, the springs bias the seal segments toward the rotor, while fluid pressure applied during operation counters the spring force so as to move the seal teeth out of engagement with, but in close proximity to the rotating component to achieve the desired sealing function.
In steam turbines, for example, it is customary to employ a plurality of arcuate seal ring segments to form a labyrinth seal about and between the stationary and rotating components. Typically, the arcuate seal ring segments are disposed in an annular groove in the stationary component (or casing), and are designed to be concentric about the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. The seal faces typically carry a radially-directed array of axially spaced teeth that are radially spaced from an array of axially spaced annular grooves forming the sealing surfaces of the rotating component. Alternatively, the rotating component may have a smooth surface in radial opposition to the array of teeth on the seal faces. In any event, the sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface(s) of the rotating component.
The annular groove in the stationary component is generally dovetail-shaped, having locating flanges directed axially toward one another and defining a slot therebetween. The stationary component is split lengthwise such that the semi-annular dovetail grooves may receive correspondingly-shaped arcuate seal ring segments. More particularly, the arcuate segments are similarly dovetail-shaped with a pair of flanges directed axially away from one another for disposition within the dovetail groove, with a narrow neck joining the seal face and the flanges of the segment and passing through the slot defined by the locating flanges of the grooves. The neck carries the arcuate seal face radially inwardly of the groove when installed, i.e., the arcuate seal face is radially adjacent the rotor.
Many designs utilize springs to return and hold the seals against a stop to a designed radial clearance. While numerous spring designs have been used over the years, each has significant disadvantages. For example, flat springs used with turbine packing seals require a large amount of radial space behind the packing ring to meet spring loads and to avoid overstressing the spring during large displacements. Another disadvantage is the fact that each packing ring segment is contacted in only one location, i.e., at the mid point of the seal segment. In this condition, the packing segment can rotate about that pivot point with relatively small input forces. If a cyclic force were applied to the packing ring, it would be possible to create a vibratory mode which could lead to high cycle fatigue of the packing ring and/or spring. Finally, the current flat spring design allows for the application of a single spring constant only.
An alternative approach to flat springs is the use of coil springs applied at two or more locations in each segment. Coil springs decrease the amount of free space behind each packing segment to a minimum, equal to the desired segment travel, and prevent pivoting about the spring contact points, since each segment is supported at multiple points. However, the coil spring design requires multiple cylindrical pockets milled into the back of each seal segment. These milled pockets may interfere with other hardware that may be installed into the segment. In addition, variable spring rates are not easy to attain without nesting multiple coil springs. It may be desirable to control the spring rate, but that may unload one of the nested springs, allowing it to vibrate and become damaged.